A standard boat drive has an engine and a variable-pitch propeller rotated by the engine about a drive axis that is the same as the boat displacement direction. A plurality of blades projecting radially from a hub of the propeller can rotate about respective blade axes perpendicular to the drive axis. Normally a low-pitch setting, with the generally flat blades almost in a plane perpendicular to the drive axis, is used for high-speed travel while for lower-speed travel a much steeper pitch is employed, with the blades extending at a large acute angle to the plane.
The standard internal-combustion marine engine, which is often a diesel, has a drive curve with a relatively narrow rpm (rev/min) range in which it is highly efficient. The beauty of a variable pitch propeller in combination with such an engine is therefore that it can be set to transmit the maximum force to the water while allowing the engine to continue to run at its most efficient speed. This eliminates the need for a transmission between the blade and the propeller.
Thus as described in German patent 3,901,137 filed 17 Jan. 1989 by P. Mueller et al it is standard to connect a double-acting hydraulic actuator via a linkage with the blades of the propeller. A control system compares the actual engine speed with a preferred speed and is connected to a four-way valve between the compartments of the actuator and both sides of a pump. When the actual motor speed exceeds the desired set point, one of the compartments is pressurized and the other depressurized to increase the pitch and, therefore, the torque the water will exert on the propeller, and when the actual motor speed falls below the set point the compartments are oppositely connected to decrease the pitch of the propeller.
Such a system is fairly complex. Its many hydraulic lines and valves are prone to failure and need frequent servicing, especially in the harsh environment of a watercraft. Furthermore the chambers of the double-acting cylinder are continuously maintained under pressure so that the control system is, in effect, working at all times, even when the blade angle is perfect so that the torque with which the propeller resists rotation exactly equals the engine output torque.